Aiden's Explanation of Steam Engines

In this reflection, I will attempt to explain how a steam engine works. I feel this is important to learn because, after all, it is this innovation that kicked off the Industrial Revolution.

The main idea of the steam engine is to convert a pressure difference (caused by boiling water) into work (defined as force times distance). For starters, force is defined as mass times acceleration – what makes work different is the energy that is transferred in physically moving the object. You can apply as much force as you want to a wall, but the work will be zero because it never moves. Anyway, a liquid is considered a fluid in which it can never stay still when some amount of force is applied – it is always "flowing" out. Because of this, the liquid (or gas) equally (in all directions) exerts pressure outward. Without this, the fluid will naturally balance the pressure out, from high pressure (there is too much force) to low pressure (there isn't enough force).

Now imagine this: a vessel filled with water (with heat below), causing the liquid to boil and expand to gas. This gas will exert more pressure because liquid is considered to be more tightly packed (due to hydrogen bonding) than gas (London Dispersion forces). This vessel is connected to the pipe, which is connected to the steam chest.

Now this part may be the most confusing, so I'll slow down. There are two spaces in this steam chest, one top and one bottom. The top space is connected to that pipe, and as gas expands, it will be under high pressure. Now, there are three pathways (left, which connects the top to the bottom; middle, which connects to either the left or right valve connected to the exhaust (open, low-pressure space); and right, which connects the top to the bottom) that connect the top space to the bottom space (considered the cylinder with the piston). Then there is the valve, shaped like a D (rotated counterclockwise 90 degrees). This valve is placed on the top space, blocking two of the three pathways. This means that at any time, either the left or right pathway opens, allowing the high-pressure steam to expand into the cylinder while the exhausted steam moves into the other pathway, into the middle. This is the mechanism that allows steam to push the piston – when the pressure expands downward to the left pathway, the piston will be pushed to the right[1], and when it's exhausted, the valve will close the left pathway (opening the right pathway) that pushes the piston back to the left[2]!

How does the valve know when to slide at the correct time? Well, for the sake of a brief explanation, the piston is connected to a rod and to the flywheel at a radius R from the flywheel's center. An eccentric crank is attached to the same point, which it also drives, that second rod that controls the valve's movement. I want to understand why this works the way it does, and I'll explain it in my next reflection. Regardless, that is the entire mechanism of the steam engine, and ingenuity here is quite timeless.

Notes

1. It's important to clarify that as the fresh steam pushes the piston to the right, the air isn't trapped. It escapes through the right valve connected to the middle valve, to the open space (low pressure).
2. Similarly, the steam on the left of the piston escapes through the middle valve.